Native speaker here.
Honestly, I have trouble thinking of a fitting equivalent. None I can think of are close to the meaning, AND sound right coming from someone who is actually in the line of fire.
I would recommend "Feuer!" (fire), "Schiess!" (shoot, singular), or "Schießt!" (shoot, plural); the latter two could be varied by repeating them ("Shoot! SHOOT!!!"), or adding "schon" or "endlich" after it (now I can't think of a proper English equivalent. basically "Hurry up!").
For the sake of completeness, hoping not to offend anyone...
Nehmen Sie sie heraus! - implies that they are inside something, where they shall be taken out of.
Nehmen- 'EM heraus! - same, but the translation program didn't recognize "Em"
Schnell schnell schnell - correct, but simply means "Hurry up!"
Nehmen Sie sie aus - Gut them. (yes, like "eviscerate")
Schiesse - extremely formal, sounding more appropriate for the 19th century than WWII
Nehm' sie aus - dialect (should be "nimm") for the singular of "gut them"